Communications systems may utilize one or more than one transmitter to define a system coverage area. Every radio frequency (RF) transmitter has an associated geographical reception area within which communications devices, such as RF receivers, tuned to the appropriate frequency can receive transmissions from the RF transmitter. As the communications device increases its distance from the transmitter, reception on the assigned frequency becomes more difficult until, eventually, the device is out of range of the transmitter. Likewise, devices located outside a system coverage area will be out of range of the communications system.
The RF receiver will not receive any signals while out of range. Therefore, it is desirable that an RF receiver user be notified when the receiver is out of range. If the RF receiver is, for example, a selective call receiver and the user is expecting an important message, notification of out of range will indicate that the user needs to return to the network coverage area or access alternative methods for receiving messages.
Presently, RF receivers which receive a signal having a predetermined signalling protocol use the lack of detection of the predetermined signalling protocol to determine whether the receiver is out of range. Yet, several communications systems having various signalling protocols may share a single RF channel or frequency. Therefore, the out of range signal may erroneously be generated during the reception of signalling protocols of other communications systems even when the user is within the system coverage area because the predetermined signalling protocol is not received.
In addition, when the receiver is carried into a shielded area such as an elevator or a subway or even the shadow of a building, the out of range signal will be generated even if the user is still within the system coverage area because the predetermined signal is not periodically received.
Thus, what is needed is a method and apparatus for reliably detecting if a selective call receiver is within or out of the geographical coverage area of its transmitter.